Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semi-conductor wafer during the fabrication of semi-conductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. Nos. 4,141,180 to Gill, Jr., et al.; 5,205,082 to Shendon et al; and, 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan.RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan.RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
In the polishing operation shown in FIG. 1B, the slurry solution 24 must be pushed into an interface between the wafer 10 and the polishing pad 12 in order for the chemical reaction and the mechanical removal process to operate efficiently. Since the surface of a silicon wafer is a hard surface and the surface of the polishing pad is normally formed of densely packed fibers, it is difficult to ensure an abundant supply of the slurry solution at the interface between the wafer and the polishing pad. Various techniques have been proposed to improve the supply of the slurry solution into the interface. Two of such techniques are shown in FIGS. 2A, 2B, 3A and 3B. FIGS. 2A and 2B show a technique in which a perforated polishing pad 28 is utilized. The perforated polishing pad 28 is formed with a multiplicity of perforations 30 through the pad thickness. As shown in FIG. 2B, typically, a perforation having a diameter of 0.075 in and a height of 0.05 in (i.e., through the complete thickness of the hard pad 32) is used. Alternatively, a more popularly used technique is to provide a grooved polishing pad 34 as shown in FIG. 3A. In the grooved polishing pad 34, grooves 36 are provided in a surface layer 38 of the hard pad. As shown in FIG. 3B, a typical groove is formed with a width of 0.01 in and a depth of 0.015 in, while the groove-to-groove distance is about 0.06 in. It should be noted that the perforations 30 and the grooves 36 are formed only through or in the hard pad layer and not into the soft pad layer.
While the perforated pad or the grooved pad shown in FIGS. 2A.about.3B provide some improvement over conventional polishing pads that have no surface modifications, the improvement is limited and the uniformity of the surface polishing is still less than ideal. It has been noticed that even though provisions have been provided on the polishing pad surface, the opposing surfaces of the wafer and the surrounding retaining ring are still hard and solid surfaces. The feeding of the slurry solution into the interface between the wafer and the polishing pad is therefore still difficult and limited.
It is therefore an object of the present invention to provide a slotted retaining ring for a CMP polishing head that does not have the drawbacks or shortcomings of the conventional retaining ring for such polishing head.
It is another object of the present invention to provide a slotted retaining ring for a CMP polishing head which has a plurality of slot recesses formed in a bottom surface of the retaining ring.
It is a further object of the present invention to provide a slotted retaining ring for a CMP polishing head that is constructed of a torroidal ring member that has parallelly situated planar top and bottom surfaces wherein the bottom surface is provided with a plurality of recesses.
It is another further object of the present invention to provide a slotted retaining ring adapted for holding a CMP head wherein a plurality of slot recesses are provided on a bottom surface of a retaining ring which are in a tapered shape.
It is still another object of the present invention to provide a slotted retaining ring adapted for holding a CMP head which has a plurality of recesses formed in the shape of trapezoidal shape with wide base portion adjacent to an outer periphery of the retaining ring.
It is yet another object of the present invention to provide a slotted retaining ring adapted for holding a CMP head that has a plurality of slot recesses in the bottom surface of the ring provided in trapezoidal shape with a tip portion adjacent to and spaced apart from an inner periphery of the retaining ring.
It is still another further object of the present invention to provide a method for chemical mechanical polishing a semiconductor wafer by using a polishing head equipped with a slotted retaining ring by first providing a plurality of trapezoidal shape slot recesses in a bottom surface of the retaining ring adapted for holding the polishing head.
It is yet another further object of the present invention to provide a method for chemical mechanical polishing a semiconductor wafer by using a polishing head which is equipped with a slotted retaining ring by first providing a plurality of slot recesses in a bottom surface of the ring and then rotating the retaining ring in a direction such that a wide base portion of the recess is advanced toward a slurry solution for transporting slurry solution toward the wafer to be polished.